To place heat-shrink sleeves on traveling articles, it is conventional to use a technique whereby the sleeves are cut from a continuous sheath that passes over a sheath-opening shaper, which shaper is held floating by co-operation between outer wheels and backing wheels of parallel axes carried by the shaper, which outer wheels serve to cause the sheath to advance along the shaper (which is generally vertical) up to and beyond cutter means. Other wheels are generally provided downstream from the cutter means to eject the segment of sheath that has been cut off onto the article that travels to a position vertically beneath the shaper.
Thus, in most of the techniques used, there are first outer wheels for advancing the sheath over the shaper, and second outer wheels serving to eject the cut-off sheath segments onto the articles in question. All of the outer wheels are naturally motor-driven, and the way they are motor-driven has given rise to various types of arrangement.
Thus, proposals have been made for the motor drive of the first and second wheels to be completely independent so as to enable the second wheels to turn much faster than the first, thereby causing the cut-off sheath segment to drop vertically more quickly onto the article in question. That approach is illustrated in document EP-A-0 109 105. In another approach, the rotary drive of the first and second wheels is synchronized, as shown in document EP-A-0 000 851.
Nevertheless, it has been found that the above-mentioned techniques impose limits in terms of rates of throughput, since when high rates are reached, it is found that the sheaths are frequently poorly positioned on the articles, particularly when they constitute sleeves of considerable height.
More recently, an important advance has been made by a technique implementing synchronous control over the electric motors concerned by means of a common electronic programmer arranged to determine a continuous speed variation profile so as to control the ejection of each sheath segment, said programmer including at least one control card that co-operates with an adjacent coder mounted at the end of a shaft that is driven in rotation by a central motor and gearbox unit. This is illustrated in document WO-A-99/59871 in the name of the Applicant. According to that technique, the synchronization makes it possible to envisage rates of throughput that are higher than before, and this is possible with sleeves of a diameter that is hardly any greater than the maximum diameter of the articles.
Nevertheless, there is an increasing demand for ever higher rates of throughput, commonly reaching values of 300 to 600 strokes per minute.
It is then preferable to use machines that are further improved, abandoning the system whereby articles advance stepwise, and also abandoning the coder system mounted at the end of a shaft driven in rotation by a central motor and gearbox unit (as described in above-mentioned document WO-A-99/59871), and instead to make use of a virtual shaft common electronic programmer for controlling all of the electric motors, with the instruction for ejecting a cut-off sheath segment being given by a cell when the traveling article goes past it.
In parallel with this search for very high rates of throughput, there is also a trend to use sheaths made of heat-shrink film that is of ever smaller thickness. As an indication, conventional techniques used to use heat-shrink films with thickness of the order of 50 micrometers (μm), whereas nowadays it is desired to use films of heat-shrink plastics material that is of smaller thickness, i.e. possibly as little as 25 μm, and that is also of smaller density.
The two above-mentioned requirements thus considerably complicate organizing sleeve-placing devices, and mention can be made of one type of technical problem that is becoming more and more awkward, and this relates to positioning the sheath on the shaper at the time it is being held stationary for the cutting and ejection pass.
The slightest variation in the position of the downstream portion of the sheath on the shaper, i.e. the portion that is to constitute the segment for ejection after the cutting pass, has the effect of giving greatly-varying heights to the cut-off sheath segment (where the height of a segment is measured along the direction of the generator lines of said segment).
In above-mentioned document WO-A-99/59871, proposals are made to arrange the control of the electric motors associated with the first wheels for advancing the sheath and with the second wheels for ejecting the cut-off segment in such a manner that the continuous profiles of speed variation for said motors are bell-shaped, rather than being squarewave-shaped was the previous practice. Nevertheless, it has been found that by causing such a device to operate at very high rates of throughput, the above-mentioned bell-shaped curves give rise to problems in terms of accuracy at the end of the advance/eject cycle, even when adopting a virtual-shaft common electronic programmer.
This lack of accuracy is also to be found when the sleeve-placing device is stopped, prior to a new period of operation, such that the position of the sheath is de facto inaccurate when operation restarts.
A direct consequence of this is that the axial position of the sheath on the shaper is never defined with complete accuracy. In practice, the free edge of the sheath is held stationary a little upstream from the second wheels for ejection and it is the following thrust from the sheath that brings this free edge into contact with the ejection wheels for the ejection process. The height of the segment is never accurately constant and it is never possible to completely eliminate the risk of the sheath slipping on the shaper at the time it is being held stationary for the cutting and ejection pass.